Some of the prior art focusing methods of a large aperture zoom lens include a foremost lens focusing system in which the first one of groups of lens pieces in series is displaced for focusing. The foremost lens focusing system has a considerable diameter of the lens group that is to be displaced for focusing and accordingly has an increased weight, and such a system is inappropriate for quick focusing. This focusing method is disadvantageous in its poor operability because displacement of the lens pieces during focusing greatly varies a weight balance of the zoom lens. In addition, when the zoom lens at the telephoto end renders the close object in focus, spherical aberration tends to be under-corrected, which is hard to compensate for.
Some other prior art focusing methods of the large aperture zoom lens include a foremost lens inner-focusing system in which the zoom lens has its first one of groups of lens pieces in series functionally divided into two, namely, front and rear subsets of lens pieces, and the rear subset is displaced for focusing. In contrast with the aforementioned foremost lens focusing system, the foremost lens inner-focusing system is more advantageous in that it can relatively easily suppress the variation in spherical aberration when the zoom lens at the telephoto end renders the close object in focus although it still has difficulty in fully compensating for the aberration. Regretfully, a drastic reduction in weight of the lens subset that is to be displaced for focusing is unattainable, and hence, such a focusing system is not suitable for quick focusing.
Further some other prior art focusing methods of the larger aperture zoom lens include an inner-focusing system in which the second one and/or the further succeeding one(s) of groups of lens pieces is displaced for focusing. Such a focusing system generally enables the zoom lens to reduce its diameter, which makes the zoom lens suitable for quick focusing and well-balanced in weight during the focusing operation. However, the inner-focusing system is disadvantageous in that the greater a diameter of the first group of lens pieces is, the more it has difficulty in suppressing the variation in the aberration when the zoom lens renders the close object in focus.
The list of the prior art documents cited hereinafter is provided below:
Patent Document 1
    Japanese Patent No. 4401469Patent Document 2    Preliminary Publication of Unexamined Japanese Patent Application No. 2007-212830Patent Document 3    Preliminary Publication of Unexamined Japanese Patent Application No. 2009-086535Patent Document 4    Preliminary Publication of Unexamined Japanese Patent Application No. 2009-156893Patent Document 5    Preliminary Publication of Unexamined Japanese Patent Application No. H07-013079Patent Document 6    Japanese Patent No. 3564061
Some exemplary prior art large aperture telephotographing zoom lenses already disclosed include that which comprises the foremost or first lens group of positive refractivity positioned the closest to the imaging plane, the succeeding second lens group of negative refractivity, the third lens group of positive refractivity, and the rearmost or fourth lens group of positive refractivity arranged in series, and employs the front lens inner-focusing system in which the first lens group is divided into two, namely, front and rear subsets of lens pieces, and the rear subset are displaced for focusing (e.g., see Patent Documents 1 and 2).
Such a prior art large aperture telephotographing zoom lens has the lens subset that is to be displaced for focusing is heavy as already mentioned and hence is not suitable for quick focusing. Also, the zoom lens is not a solution to the problem that spherical aberration tends to be under-corrected especially when the zoom lens at the telephoto end renders the close object in focus.
Some other prior art large aperture zoom lenses already disclosed include that which comprises the foremost or first lens group of positive refractivity positioned the closest to the object, the succeeding second lens group of negative refractivity, the third lens group of positive refractivity, and the rearmost or fourth lens group of positive refractivity arranged in series, and employs the inner-focusing system in which the third lens group is displaced for focusing (e.g., see Patent Documents 3 and 4).
Such a prior art large aperture telephotographing zoom lens is capable of quick focusing. However, the focusing operation by the third lens group of positive refractivity permits astigmatism to vary to be more under-corrected during a transition from a state of the infinity point in focus to another state of the close object in focus, especially, when the zoom lens takes a posture at the wide-angle end, and thus, such a system is disadvantageous in that the astigmatism cannot be fully compensated for when the zoom lens renders the close object in focus.
Still some other prior art large aperture telephotographing zoom lenses already disclosed include that which comprises the foremost or first lens group of positive refractivity positioned the closest to the object, the succeeding second lens group of negative refractivity, the third lens group of positive refractivity, and the rearmost or fourth lens group of positive refractivity arranged in series, and has its first to third lens groups cooperatively combined to serve as an afocal system and its fourth lens group isolated as a master lens group for imaging (e.g., see Patent Document 5). The fourth lens group has a front lens subset 4A of positive refractivity positioned the closest to the object, a middle lens subset 4F of negative refractivity, and a rear lens subset 4B of positive refractivity arranged in series, and the rear lens subset 4F is displaced for focusing. This focusing lens subset 4F includes a composite lens having positive and negative lens pieces cemented together, which is effective in both downsizing and weight reduction of the zoom lens to such an extent of enabling quick focusing.
Similar to the large aperture telephotographing zoom lenses disclosed in Patent Documents 3 and 4, however, the one as disclosed in Patent Document 5 considerably develops curvature of field when the zoom lens at the wide-angle end renders the close object in focus, and thus, its performance of imaging is unsatisfactory. In general, as to the focusing operation by the fourth lens group serving as the master lens group, the greater a diameter of the first lens group is, the harder it is for the zoom lens to compensate for the variation in astigmatism when the zoom lens renders the close object in focus.
Yet some other prior art large aperture telephotographing zoom lenses already disclosed include that which comprises the foremost or first lens group of positive refractivity positioned the closest to the object, the succeeding second lens group of positive or negative refractivity, the third lens group of negative refractivity, and the rearmost or fourth lens group of positive refractivity arranged in series, and has its second lens group displaced for focusing (e.g., see Patent Document 6).
Such a zoom lens has its second lens group as a focusing lens displaced toward the image plane during varying the magnification for zooming from the wide-angle end to the telephoto end, so that the axial light flux incident on the entrance pupil passes closer to the optical axis when the zoom lens takes a posture at the telephoto end, and unlike the aforementioned foremost lens inner-focusing system, this system enables the zoom lens to have its focusing lens slightly downsized. In this manner, however, the resultant zoom lens is unsatisfactory in downsizing and reduction of its weight. The focusing operation by the second lens group permits astigmatism to vary to be more over-corrected when the zoom lens is in transition from a state of the infinity point in focus to another state of the close object in focus throughout the entire zoom range. When it renders the close object in focus, the zoom lens is not able to satisfactorily compensate for the variation of astigmatism that tends to be over-corrected.